Since polyamide resins have excellent mechanical and thermal properties and oil resistance, they are broadly used in various parts of, for example, automobiles and electric and electronic products. Also, since reinforced polyamide resins in which the polyamide is blended with an inorganic filler such as glass fiber show remarkably improved properties such as mechanical characteristics, heat resistance and chemical resistance, it becomes possible to use them as substitutes for certain parts conventionally made of metals, for the reasons, for example, of the weight reduction of products and rationalization of production steps, so that such resins have been positively studied in recent years.
Particularly, they are frequently used in the form of black colored molding articles in the case of their use in outdoor applications. Because of this, blending of carbon black is employed as a means to obtain black colored molding articles most inexpensively while easily improving weather resistance.
However, it is known that compositions prepared by merely blending polyamide with carbon black have many problems.
For example, it has been reported that certain problems such as reduction in toughness could occur in products obtained from a composition in which polyamide containing no inorganic filler was blended with carbon black. As a means for solving such problems, several techniques in which a specific dyestuff is jointly used with carbon black have been disclosed. For example, a composition for molding use in which a polyamide resin is blended with carbon black and nigrosine (JP-B-60-43379; the term "JP-B" as used herein means an "examined Japanese patent publication) and a method in which a thermoplastic resin containing nylon is colored by blending it with carbon black and a specific copper-phthalocyanine dyestuff (JP-B-6-35540, JP-B-6-35541) have been reported.
However, these disclosed techniques do not suggest or disclose about a method for improving surface appearance of molding articles when an inorganic filler is blended, or a method for inhibiting fading of black color in rain-accompanying weather resistance test. These problems concerning appearance and weather resistance occur in many cases, particularly when polyamide contains an inorganic filler in a relatively high concentration.
On the other hand, it has been reported that certain problems such as remarkably reduced surface gloss of produced molding articles occur when polyamide containing an inorganic filler is blended with carbon black. As a means for resolving such problems, the use of a resin composition in which polyamide containing glass fiber and carbon black is further blended with nigrosine has been disclosed (JP-A-4-370148; the term "JP-A" as used herein means an "unexamined published Japanese patent application").
However, according to the studies carried out by the inventors of the present invention, countless cracks were formed on the surface of molding articles when these molding articles were obtained by the disclosed method and subjected to rain-accompanying weather resistance test. In addition, exposure of glass fibers to the surface was marked particularly when the glass fiber content was relatively high, so that their use in the field could cause serious problems.
As described above, the conventional techniques do not suggest or disclose a black colored reinforced polyamide resin composition having such properties that molding articles obtained therefrom show excellent surface appearance, have mechanical characteristics substitutive for metals and hardly cause fading of black color under working conditions in the field, particularly where they are exposed to rain.